In modern data communications, for example, communications over power lines, coaxial cables, and wireless, information bits are grouped and encoded into data packets. A packet-based communication network can more efficiently utilize a shared medium. A structure for a typical data packet 100 is shown in FIG. 1. The data packet 100 includes a preamble 102, a packet header 104, and one or more packet payloads 106. The preamble 102 serves to achieve various functions, such as packet detection, symbol boundary detection, and channel estimation.
Turning to FIG. 2, the preamble 102 includes a sequence 201 of preamble symbols. The preamble symbols are often designed with various characteristics to promote the above-mentioned functions. Examples of such characteristics include a flat frequency response, a low peak to average power ratio, and high uniqueness (e.g., the symbol may be a unique word or pattern). Preferably, the preamble symbol is selected to be more easily discoverable under heavily distorted channel conditions and strong noise. In practice, the preamble often includes a polarity transition. In some scenarios, all preamble symbols of the sequence 201 are identical, with the exception that the last one or two preamble symbols are negated. In the example of FIG. 2, the sequence 201 includes six positive symbols 202 and two negative symbols 204 that are separated by a transition boundary 206 (e.g., a transition in a received signal for the preamble 102). The transition boundary 206 may be used to establish a reference point for locating the packet header 104 that follows the preamble 102. Other arrangements of positive symbols and negative symbols are used in various embodiments.
In a conventional receiver, the preamble 102 can be detected by one or more correlators. Two forms of correlators are commonly used: an auto-correlator 300 and cross-correlator 400 as respectively illustrated in FIG. 3 and FIG. 4. The auto-correlator 300 and cross-correlator 400 are temporal correlators. The auto-correlator 300 includes a delay component 302, a multiplier 304, and an accumulator 306. The auto-correlator 300 receives an input signal s(n) and provides the input signal s(n) to the delay component 302 and to the multiplier 304. The delay component 302 introduces a delay of N units to the input signal s(n) (e.g., a duration of time or number of clock signals). The delay component 302 provides a corresponding delayed signal s(n−N) as a reference signal to the multiplier 304, as shown in FIG. 3. The multiplier 304 multiples the signals s(n) and s(n−N) and provides a corresponding multiplied signal to the accumulator 306. The accumulator 306 accumulates the multiplied signal over a predetermined duration, typically a same duration as a preamble symbol period (e.g., a predetermined duration of the preamble 102). The accumulator 306 provides a temporal auto-correlation output signal AR(n).
FIG. 4 shows the cross-correlator 400, in which a delayed input signal is correlated with various local reference signals. The cross-correlator 400 includes a plurality of N delay components 402, a corresponding plurality of N multipliers 404, and an accumulator 406. The cross-correlator 400 receives the input signal s(n) and provides the input signal s(n) to the plurality of N delay components 402. The plurality of N delay components 402 are cascaded to provide respective output signals s(n−1), s(n−2), s(n−3), . . . and s(n−N) to the plurality of N multipliers 404. The plurality of N multipliers 404 also receive a plurality of N reference signals r(0), r(1), r(2), r(3), . . . and r(N−1) and thus provide corresponding multiplied signals to the accumulator 406. The accumulator 406 provides a temporal cross-correlation output signal CR(n).
Both the auto-correlator 300 and the cross-correlator 400 can detect the preamble 102 by detecting a peak in their respective output signals. However, when the preamble 102 is distorted by narrow-band interference or a single-tone interference signal, both correlators may trigger a false alarm and thus claim a detection of a preamble (e.g., preamble 102) when a preamble is not actually present. Narrow-band interference and single-tone interference signals are commonly found in power line channels.